Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field B0 whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system to which the measurement is related. The magnetic field B0 produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency).
From a macroscopic point of view, the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the corresponding magnetic field B1 of this RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the magnetization is deflected from the z axis to the transverse plane (flip angle 90°).
The transverse magnetization and its variation can be detected by means of receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis.
In order to realize spatial resolution in the body, constant magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field B0, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body.
The signal data obtained via the receiving coils correspond to the spatial frequency domain and are called k-space data. The k-space data usually include multiple acquired k-space profiles (lines in k-space) of different phase encoding. Each k-space profile is digitized by collecting a number of samples. A set of k-space data is converted to an MR image by means of Fourier transformation.
In a variety of MRI applications, motion of the examined patient can adversely affect image quality. Acquisition of sufficient MR signals for reconstruction of an image takes a finite period of time. Motion of the patient during that finite acquisition time typically results in motion artifacts in the reconstructed MR image. In conventional MR imaging approaches, the acquisition time can be reduced to a very small extent only, when a given resolution of the MR image is specified. In the case of medical MR imaging, motion artifacts can result for example from cardiac and respiratory cyclic motion, and other physiological processes, as well as from patient motion resulting in blurring, misregistration, deformation and ghosting artifacts.
Different approaches have been developed to overcome problems with respect to motion in MR imaging. Among these is the so-called PROPELLER imaging technique. In the PROPELLER concept (Periodically Rotated Overlapping ParalEL Lines, see James G. Pipe: ‘Motion Correction With PROPELLER MRI: Application to Head Motion and Free-Breathing Cardiac Imaging’, Magnetic Resonance in Medicine, vol. 42, 1999, pages 963-969), MR signal data are acquired in k-space in N strips, each consisting of L parallel k-space lines, corresponding to the L lowest frequency phase-encoding lines in a Cartesian-based k-space sampling scheme. Each strip, which is also referred to as k-space blade, is rotated in k-space by an angle of, for example, 180°/N, so that the total MR data set spans a circle in k-space. If a full k-space data matrix having a diameter M is desired, then L and N may be chosen so that L×N=M×π/2. One essential characteristic of PROPELLER is that a central circular portion in k-space, having a diameter L, is acquired for each k-space blade. This central portion can be used to reconstruct a low-resolution MR image for each k-space blade. The low-resolution MR images are compared to each other to detect in-plane displacements and phase errors, which are due to patient motion. A suitable technique such as cross-correlation is employed to determine which k-space blades were acquired with significant motion-induced displacement or include other types of artifacts. As the MR signal data are combined in k-space before the reconstruction of the final MR image, the MR data from k-space blades are weighted according to the artifact level detected by cross-correlating the k-space blades, so that artifacts are reduced in the final MR image. The PROPELLER technique makes use of oversampling in the central portion of k-space in order to obtain an MR image acquisition technique that is robust with respect to motion of the examined patient during MR signal acquisition.
However, drawbacks of the known PROPELLER approach result from the fact that its application is restricted to the specific circular acquisition of k-space profiles from a number of successively rotated blades. The highly effective motion-compensation and motion-correction concept of the PROPELLER approach is not compatible with Cartesian k-space sampling schemes.
As an alternative, the so-called navigator technique has been developed to overcome problems with respect to motion by prospectively adjusting the imaging parameters, which define the location and orientation of the volume of interest within the imaging volume. In the navigator technique hereby, a set of navigator signals is acquired from a spatially restricted volume that crosses, for example, the diaphragm of the examined patient to determine the breathing motion of the patient. For registering the navigator signals, so-called 2D RF pulses may be used. These excite the spatially restricted navigator volume, for example of pencil beam shape, which is read out using a gradient echo. Other ways to detect the motion-induced momentary position of the volume of interest is the acquisition of two-dimensional sagittal slices that are positioned at the top of the diaphragm, or the acquisition of three-dimensional low-resolution data sets. The respective navigator volume is interactively placed in such a way that a displacement value indicating the instantaneous position of the moving anatomy can be reconstructed from the acquired navigator signals and used for motion correction of the volume of interest in real time. The navigator technique is primarily used for minimizing the effects of breathing motion in body and cardiac exams where respiratory motion can severely deteriorate the image quality. Gating and image correction based on the MR navigator signals may be used to reduce these artifacts.
However, a drawback of the navigator technique is that additional acquisition of the navigator signals is required which results in an extension of the overall scan time. Moreover, the known navigator methods are difficult to apply if the volume of interest, from which the MR signals for imaging are to be acquired, partially overlaps with the navigator volume. If the navigator volume is (at least partly) superimposed upon the respective volume of interest, the image quality may be degraded due to incorrect detection of the motion state or due to saturation of the nuclear magnetization within the navigator volume.
The U.S. Pat. No. 6,144,874 concerns an magnetic resonance imaging method in which central k-pace vies and peripheral k-space views are acquired in a narrow and wider acquisition window, respectively. The k-space views are acquired upon first and second respiratory gating signals.